At first glance, Vanessa Redgrave is all wrong to play Joan Didion. She's square and sturdy - almost regal; the author is small-framed and fragile; she looks as if a strong breeze would blow her to the ground. But when Didion's words pour forth from Redgrave's expressive mouth, it's impossible to imagine another actress in the role.

'This happened on 30 December, 2003. That may seem a while ago, but it won't when it happens to you.

'And it will happen to you. The details will be different, but it will happen to you.

'That's what I'm here to tell you.

'You see me on this stage, you sit next to me on a plane, you run into me at dinner, you know what happened to me.

'You don't want to think it could happen to you.

'That's why I'm here.'

Redgrave is the reason The Year of Magical Thinking - Didion's own dramatic retelling of her best-selling memoir chronicling the aftermath of the sudden death of her husband, novelist John Gregory Dunne, and the mysterious illness of her daughter Quintana - is not only the hottest ticket in town, but also the theatrical event of the season, and rightly so (the show is directed by David Hare and designed by fellow Brit Bob Crowley). The legendary British actress's Broadway appearances have been few and far between: she made her debut in 1976 with Ibsen's The Lady From the Sea (directed by her ex-husband Tony Richardson); she returned in 1989 for a triumphant turn in Tennessee Williams's Orpheus Descending; and in 2003 she took on Eugene O'Neill's Long Day's Journey Into Night, and was rewarded with a Tony for her earthy portrayal of morphine-addled matriarch Mary Tyrone. When Redgrave steps on to one of our stages, New Yorkers take notice. Tourists take notice. Doormen and delivery boys take notice.

And from the moment the lights come up, we can't take our eyes off her. There's no true set to speak of - a couple of sky-hued lights meant to evoke spots like Malibu (where Didion and her family kept a home). A pale wooden Adirondack-style chair is positioned at centre-stage. Redgrave reclines, leans forward thoughtfully, collapses into herself, and, once in a while, gets up, perhaps just to stretch her legs. The mood couldn't be more relaxed... or the subject matter more nerve-racking. Hare knows that no theatrical accoutrement is required. A few dramatic lighting flourishes notwithstanding, all that's needed is Didion's prose and Redgrave's Didion.

That the audience is rapt is an understatement. We are practically hypnotised by Redgrave's every movement. Every tilt of her head. The flicks of her wrists and wringing of her hands. How she lets her platinum hair down and shakes it out before pulling it again into a sensible ponytail. The way her hands calmly, neatly smooth out the wrinkles in her ankle-length skirt.

The tale Redgrave is charged with telling is as gut-wrenching as it is deeply personal. On Christmas Day 2003, Didion was preparing for dinner with her husband John. The two had spent the morning at a New York City hospital, where their grown daughter Quintana lay comatose (she had checked in five days earlier with flu-like symptoms, and subsequently entered septic shock). She built a fire. He drank a scotch. 'He was sitting across from me talking,' Redgrave/ Didion recalls. 'Then he wasn't. Wasn't talking.' He had what Didion later learnt was a massive coronary.

'Life changes fast.

'Life changes in the instant.

'You sit down to dinner and life as you know it ends.

'The question of self-pity.'

Those were the first words Didion wrote after it happened, and they are the first words of her critically adored, National Book Award-winning memoir. (The title, incidentally, refers to Didion's often logic-defying way of handling the tragedy. Not giving away her husband's shoes, for example, because he would need them if he came back. 'Magical thinking,' she termed it.) But as the novel went to press, Quintana passed away. If the book was Didion's way of dealing with Dunne's death, the play, one imagines, is her way of coping with Quintana's. It's less an adaptation of the book than an extension. But it's nowhere near as tidy or well-constructed. The show seems to come to an end just after Quintana's death ('I have a few more questions I need to ask her... When I said you're safe, I'm here, was that a lie or did you believe it?') But there are three more codas - a recollection of a Christmas with Quintana, a meditation on grief and geology, and a story of swimming with her husband and daughter - and they ultimately take some of the air out of the evening.

Still, Redgrave is so captivating that any dramaturgical flaws are easily forgiven. At one point she points to a thin gold bangle around her wrist. 'Do you remember,' she asks, 'I said I gave her a bracelet like this one?' She holds her wrist out to the audience as we imagine a similar bracelet around Quintana's wrist. She pauses, savouring the pin-dropping silence that permeates the Booth Theatre. Her voice drops ever so lightly. 'I gave her this one.' She fixes her azure eyes on the audience. They are strong and unwavering. They do not well with tears. Ours, however, do.